1. Field of the Invention
The present invention relates to a non-linear optical article or material which exhibits non-linear response when used in an optical instrument or as an optical element or tool, and particularly to such an optical article or material constituted of a derivative of benzalacetophenone and a process for the preparation of such an optical article.
Non-linear optical materials include those in which non-linear response results due to induced polarization of electrons by the electric field created by the light incident to a material. In other words, the term "non-linear optical material" means materials for optical use, which exhibit the so-called non-linear optical effect. In general, such effect is due to the phenomenon which may be indicated by the second and higher order terms in the following equation of: EQU P=.chi..sup.1 E+.chi..sup.2 E.multidot.E+.chi..sup.3 E.multidot.E.multidot.E+ - - - +.chi..sup.n E.multidot.n;
wherein P is polarizability of a material, E is intensity of the electric field, and .chi..sup.n is non-linear sensitivity of the n-th power.
Particularly, when the phenomenon known as the second harmonic generation (SHG), which is obtainable by the utilization of the secondary effect, is intended to be applied to optical processing, the incident light is converted into two light waves both having frequencies corresponding to those of the second harmonic waves. This phenomenon may be conveniently utilized for a variety of optical processing including conversion of wave length, processing of signals and modulation of laser beams.
2. Related Art Statement
Crystals of inorganic compounds, such as KH.sub.2 PO.sub.4 (KDP), LiNbO.sub.3, NH.sub.4 H.sub.2 PO.sub.4 (ADP), have hitherto been used as the non-linear optical materials. However, those known materials have several disadvantages, for instance, in that single crystals thereof having high optical purities are very expensive, that they often are deliquescent and, therefore absorb moisture from the atmosphere and thus cause problems in handling and that the non-linear sensitivities thereof do not reach satisfactory level.
On the other hand, since the utility of organic materials for such applications was reported in 1983 in a symposium of the American Chemical Society, there have been some reports which describe the use of crystals of organic compounds as non-linear optical materials. The compounds which have been already reported as having utility as non-linear optical materials include, for example, urea, aniline type compounds, and benzalacetophenones including nitro groups which are different from the compounds used in this invention. However, these known organic compounds do not exhibit satisfactory non-linear optical effects, or the compounds which exhibit relatively high level non-linear effects have cut off wavelengths that are significantly shifted onto the long wave length range to thus limit the wave length range of the light waves which can be processed therethrough.